What is the billable AWS regional data transfer via elastic IPs (PublicIP-In and PublicIP-Out)?

In the AWS bills console, I have this line item: "$0.010 per GB - regional data transfer - in/out/between EC2 AZs or using elastic IPs or ELB". In the Cost Explorer console, I see the exact same charge quantity when I filter by Usage Type Group: "EC2: Data Transfer - Inter AZ". It lists the exact usage type as "XXXX-DataTransfer-Regional-Bytes" (where XXXX is my AWS region). If I group by dimension "API operation", it breaks down the charge into two components: PublicIP-In and PublicIP-Out. I'd like to understand whether I can avoid this charge, such as by changing which datacentre (availability zone or AZ) my EC2 virtual machine is running in, or by configuring the VPC (the virtual network my VM is connected to) to route traffic differently. What causes the regional data transfer charge (PublicIP-In/Out) and is this documented anyplace official (such as an AWS Pricing page or API reference but excluding blogs)? In my case, the PublicIP-In charge is 2-3 orders of magnitude larger than the PublicIP-Out charge. My application is an EC2 (on a public subnet in a single AZ) that mirrors data from a public server to S3. The public server is not part of my account, but I suspect it could be hosted from an EC2 in the same region (possibly in the same AWS Organization). I'm not using a NAT (which would cost 6x more than this per GB), hence neither am I using an S3 gateway VPC endpoint (to bypass a NAT), so both download and upload (by the EC2) will be routed via my VPC internet gateway (IGW). My VPC flow logs (which allow me to tabulate tallies of data transfer by IP address pairs and port number pairs, using the CloudWatch Log Analytics console) suggest that either the upload (to S3) or the download (from the public server), but not both combined, would be of a scale to account for the usage volume in this line item (the regional data transfer). I had expected not to incur any data transfer charges, because ingress from the internet is ostensibly supposed to be free, and likewise for same-region traffic between EC2 and S3.